Aircraft: MEDIUM TRANSPORT; LOW WING; 2 TURBOJET ENG
Phase of flight: Cruise
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Other / Unknown

The last two days prior have been inadequate staffing wise with multiple sick outs. I've been CIC multiple times the last two days in situations where I have to put breaks at 30 or 35 minutes and one or two people on break with 6 or 7 people working right up against 2 hours on position and controllers pissed at me because of the staffing as if I created this; while I have to walk around and ask if people are willing to stay holdover for overtime; while some tell me 'Hell no I'm not staying.' In those situations; needing a D side (Radar Assist) puts a significant strain on the already fragile staffing equation. While anyone can agree that in principle we should put the bodies where they are needed; but the reality of the 'group think' in the situation is that we are now naturally working more traffic than we ever used to by ourselves when we had good staffing. So when I walk in to work today I see breaks at 30 or 35 minutes and can tell that today will be no different than the last two. Otherwise I felt good. I had adequate sleep. I love the sector I was assigned to work. It's complex and it forces me to use all of my skills. I love the problem solving.I recall around 20 something departures in the departure list which is a moderate amount which indicates the sector's potential. It became a busy session. I recall the Controller next to me answering some of my landline calls. I recall mentioning to that Controller that I might need a D side. I had [the other aircraft] and Aircraft X tied up and there was plenty other things going on all at once. I decided to break the tie between [the other aircraft] and Aircraft X and I put Aircraft X on a vector to go behind [the other aircraft]. I was proud of myself for the plan I had. It required some extra coordination but is worth it for everyone involved to not stack them and hand the problem off to Approach since they land soon. I put Aircraft X on a 030 vector; [and] started coordinating everything with everyone. I had a NORDO aircraft. Also plenty of CA (Conflict Alert) flashing I recall associated with a MOA that I also work. We are numb at that sector to flashing on the scope. Aircraft flash in conflict nearly constantly when it gets busy like that. Aircraft X notified me that he was responding to a TCAS RA (Resolution Advisory) in a descent. I noticed the VFR traffic off his left side that I was not talking to at 11;300 feet VFR I recall. I called the traffic and advised a left turn 30 degrees with their descent. The left turn was not contrary to the TCAS alert because TCAS only gives vertical guidance. In the end the turn wouldn't have helped; the aircraft needed vertical separation and the TCAS RA accomplished that. The situation resolved and I did my best to recover.I needed a D side. I mentioned to the Controller next to me about a minute prior that I probably needed one and then this happened. I don't enjoy the pressure that the short staffing brings to delay needing a D side as far as possible because of the struggle between breaks and safety. This doesn't excuse any particular situation; but it is a distraction enough to be working traffic while wondering how long I'll be plugged in. I was in the zone fully engaged with a good attitude doing the best I could to keep aircraft safely separated. This shows the safety margins; that even when we are fully engaged; missing one aircraft like this little VFR that I'm not talking to can be significant. Another set of eyes is definitely helpful. Jacksonville Center Controller reported not noticing an unidentified VFR aircraft on a converging heading with an IFR descending aircraft that responded to a TCAS RA due to high workload from lack of proper staffing at facility.